1. Field of the Invention
The present invention relates to a multiple coat metal pipe and its fabrication method in which in a metal pipe comprising a comparatively slender diameter of a pipe diameter of 20 mm or smaller which is arranged particularly at a lower portion of a chassis as a pipe of a brake pipe, a fuel pipe or the like in an automobile or the like, multiple coat layers are provided on its outer peripheral face for achieving durability against chipping (flying stone) or splashing (muddy water) in running and end working of flaring, spooling and buldging or the like is carried at a pipe end portion thereof in later steps.
2. Description of the Prior Arts
In recent years, there have been proposed a number of resin coat metal pipes for pipes of brake pipes, fuel pipes and the like arranged at a lower portion of an automobile in which coating layers are formed in a thick film shape to provide chipping resistance or splashing resistance.
As such a coated metal pipe, there have been known a metal pipe formed with a chromate coating as necessary on a Zn (zinc) or Zn/Ni (nickel) plated film that is coated on an outer peripheral face thereof, further coated with polyvinyl fluoride (PVF) having a thickness of about 20 xcexcm and coated with a thermally contractive tube further thereon thereby forming multiple coat layers and a metal pipe formed with polyamide-based resin (PA) such as PA 11 or PA 12 having a thickness of about 200 through 300 xcexcm by extrusion on a Zn or Zn/Al (aluminum) film that is coated on an outer peripheral face thereof.
However, according to the former conventional technology, there poses a problem in which a product cost is increased since the expensive thermally contractive tube is used and time and labor are needed in fabrication since the technology includes a step of heating for contracting the tube after the thermally contractive tube is covered on the polyvinyl fluoride layer which deteriorates the productively and according to the latter conventional technology, there poses a problem in which the corrosion resistance is deteriorated when the pipe undergoes attack of chipping or the like since a single layer of polyamide-based resin in a thick film shape is used.
Hence, as a result of studying in various ways in respect of a coated metal pipe having excellent corrosion resistance without deteriorating chipping resistance or splashing resistance and facilitating the fabrication, the inventors have paid attention to a metal pipe having multiple coat layers in which on an outer peripheral face of the metal pipe previously subjected to a surface treatment, a resin layer comprising a polyamide-based resin (PA), polypropylene (PP), polyethylene (PE), polyvinylidene fluoride (PVdF) or the like having adhesion force in respect of the surface treatment layer, is formed as an inner layer, on which PP, PE or polyamide-based resin similar to the above-described having chipping resistance or splashing resistance, is coated over an entire length. Further, in respect of a multiple coat metal pipe constituted as described above, a soft chipping test using stones having a size of 2.5 through 5 mm with an amount of flying stones of 500 gxc3x971 time under air pressure of 1 kgf/cm2 and a hard chipping test using stones having a size of 5 through 9 mm with an amount of flying stones of 500 gxc3x975 times under air pressure of 5 kgf/cm2, have been carried out and a result in respect of the chipping resistance has been obtained as expected by the inventors.
According to the multiple coat metal pipe constituted as described above, a satisfactory result has been obtained in respect of the corrosion resistance without deteriorating the chipping resistance or the splashing resistance as mentioned above, the following problem is posed when the pipe is actually arranged at the lower portion of an automobile or the like. That is, normally, end working of flaring, spooling, buldging or the like is carried out to mutually connect multiple coat metal pipes and in that case, an outermost layer of the multiple coat layers of the metal pipe is peeled off and thereafter, the end working is carried out. However, in the case of the above-described multiple coat metal pipe, in peeling off the outermost layer, there have been cases in which the inner layer adheres to the outermost layer and is peeled off along therewith or although the inner layer does not adhere to the outermost layer, the inner layer is partially peeled off from the coated film.
When the inner layer is also peeled off in this way, the corrosion resistance of the end working portion is deteriorated and therefore, in carrying out the peeling off operation, in order to prevent occurrence of the situation where the inner layer is peeled off, close attention must be paid and therefore, time and labor is needed in the operation, the operational performance is significantly deteriorated and the productivity cannot be improved.
The present invention has resolved such a problem and it is an object of the present invention to provide a multiple coat metal pipe and its fabrication method having excellent corrosion resistance without deteriorating chipping resistance or splashing resistance, capable of being fabricated easily and capable of simply carrying out peel off operation owing to a weak peel off strength wherein corrosion resistance of an end working portion is not deteriorated.
In order to achieve the above-described object, the inventors have carried out various researches and completed the present invention to find out that in extruding two layers of resin layers on an outer peripheral face of a metal pipe on the outer peripheral face of which a surface treatment has been carried out by an electric or molten plating process or the like, by bringing the both layers substantially into a non-adhesive state such that a peel off strength between the both layers is 75 g/cm or less, excellent corrosion resistance is achieved without deteriorating chipping resistance or splashing resistance, the peel off operation can easily be carried out and corrosion resistance at an end working portion is not deteriorated.
Therefore, according to a first aspect of the present invention, there is provided a multiple coat metal pipe which is a metal pipe constituted by multiplexing and coating a first layer comprising a resin having an adhesive force in respect of a surface treatment layer formed on an outer peripheral face of the metal pipe and capable of being extruded and a second layer provided on an outer peripheral face of the first layer, having a chipping resistance and capable of being extruded, wherein a peel off strength between the first layer and the second layer is 75 g/cm or less.
Further, according to a second aspect of the present invention, there is provided a method of fabricating a multiple coat metal pipe, wherein a first layer comprising a resin having an adhesive force is extruded on a surface treatment layer provided on an outer peripheral face of a metal pipe and a second layer comprising a resin having a chipping resistance is extruded on an outer peripheral face of the first layer and the both resin layers are brought into contact with each other and are multiplexed with each other substantially in a non-adhesive state. In this case, it is preferable that when the resins of the first layer and the second layer are provided with mutual dissolving performance, the second layer is extruded at a temperature equal to or lower than the melting point of the first layer and when the resins of the first layer and the second layer are not provided with the mutual dissolving performance, the second layer is extruded at a temperature equal to or higher than the melting point of the first layer.